Wood-rotting fungi cause losses over longer periods of time than most other diseases. Losses are caused mainly by breakage of branches and trunks due to a decline in the structural integrity of the wood. Structural integrity of the wood is altered by the enzymatic digestion of wood cell-wall components by the fungal pathogen or pathogens that have colonized the wood. Some of the same fungi that cause wood rot can also cause dieback. Learn more and see images in this article, Wood Rots in Apple Trees.
Answer provided by Alan R. Biggs, West Virginia University.